Skin plays a vital role in protecting the body against the damaging effects of sunlight, invasion by foreign microorganisms, unregulated water loss, and uncontrolled fluctuations n body temperature. Rather than serving as a passive barrier, the skin interacts actively with the immune system and serves as a major line of defense for the body. The bilayered skin equivalent (SE) provides a convenient model system for investigating the immune properties of skin cells. This simplified model of skin is constructed by layering a suspension of epidermal cells on a collagen matrix populated with fibroblasts. Skin equivalents constructed using isogeneic (Lewis) epidermal cells and allogeneic (Brown Norway) fibroblasts persist after grafting to Lewis rats, while those generated using allogeneic epidermal cells are rejected. Depletion of the Ia antigen+ Langerhans cells from the epidermal suspension used to construct the skin equivalents does not prolong the survival of the allogeneic epidermal SE grafts on rats, suggesting that keratinocytes may play a major role in provoking graft rejection. To study the interactions between grafted keratinocytes and the host immune system, SEs constructed with either isogeneic or allogeneic epidermal cell suspensions depleted of Ia antigen+ cells will be grafted to Lewis rats and the influx of Langerhans cells and leukocytes will be compared. Growth of keratinocytes on plastic substrata permanently decreases their ability to express Ia antigens. To determine whether this growth on plastic also reduces their ability to provoke graft rejection, allogeneic and isogeneic epidermal cells will maintained in defined medium on plastic dishes for at least one passage before their addition to the surface of the collagen lattices. The influx of inflammatory cells into the two sets of SE grafts will be compared to determine whether the growth of epidermal cells on the artificial plastic substratum permanently alters their ability to provoke rejection. Gamma- interferon will be added to SEs constructed with primary epidermal cells to induce Ia antigen expression on the keratinocytes, potentially accelerating graft rejection in SEs containing allogeneic, but not isogeneic, epidermal cells. A better understanding of the role of the keratinocyte in graft rejection should improve the techniques for skin expansion of the in vitro.